Unbonded flexible pipes for offshore transportation are well known in the art. Such pipes comprise an inner liner which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armouring layers on the outer side of the inner liner (Outer armouring layer(s)). The flexible pipe may comprise additional layers such as one or more inner armour layers to prevent the collapse of the inner liner. Such inner armouring layer or layers are normally referred to as a carcass. An outer sheath may be provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armour layers.
Typical unbonded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. No. 6,123,114 and U.S. Pat. No. 6,085,799.
The term “unbonded” means in this text that at least two of the layers including the armouring layers and polymer layers are not bonded to each other. In practice the pipe will comprise at least two armouring layers, which are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
The above-mentioned type of flexible pipes is used, among other things, for off shore as well as some on-shore applications for the transport of fluids and gases. Flexible pipes can e.g. be used for the transportation of fluids where very high or varying water pressures exist along the longitudinal axis of the pipe, such as riser pipes which extend from the seabed up to an installation on or near the surface of the sea, pipes for transportation of liquid and gases between installations, pipes which are located at great depths on the seabed, or between installations near the surface of the sea.
In traditional flexible pipes, the one or more outer armouring layers are most often in the form of helically wound steel wires e.g. shaped as profiles, where the individual layers may be wound at different winding angle relative to the pipe axis. The carcass is typically made from wound stainless steel strips.
A pipe of the above type needs to fulfil a number of requirements. First of all the pipe should have high mechanical strength to withstand the enormous forces it will be subjected to during transportation, laying down and in operation. The internal pressure (from inside of the pipe and outwards) and the external pressure (from outside of the pipe) are very high and may vary considerably. Simultaneously the flexible pipe should be very resistant to corrosive fluids. Furthermore, it is important that the weight is kept sufficiently low as a too high weight may rupture the pipe during laying out, and further the more weight the more expensive the transportation.
In general such pipes are expected to have a lifetime of 20 years in operation.
In order to fulfil these requirement in the prior art flexible pipes, the armouring layers or at least some of the armouring layers were made by high corrosion resistance and/or high strength steel compositions: WO 2004/079028 suggests the use of a stainless steel alloy for offshore use. The steel alloy is selected due to its high corrosion resistance and good mechanical properties.
A commercial product Duplex 2205 marketed by Outokumpu is an embodiment of the disclosure of WO 2004/079028.
The steel alloy of WO 2004/079028 is, however, very expensive due to the high amount of expensive alloy components, such a nickel. The steel alloy of WO 2004/079028 thus comprises at least 4.9% by weight.
In WO 0227056 a steel alloy is suggested for inter alia offshore use, in particular for uses where corrosion conditions are milder. This steel alloy has a low amount of nickel, however, this low nickel content has been compensated by addition of other components. Thus, instead of the nickel, the steel alloy comprises at least 10 Ni-equivalents (Nieq=Ni+0.5Mn+30(C+N)+0.5(Cu+Co)).
Co and Cu are relatively expensive alloy components, Mn is known to reduce the corrosive resistivity and should therefore not be present in too large amounts, i.e. it should not be present in an amount above 6% by weight. The amount of carbon should also not be too large, that means not above 0.5% by weight since a larger amount may result in precipitate of carbides which may result in intercrystalline corrosion. The amount of nitrogen should also be kept low, since nitrogen is known to generate porosity in the material, which may be fatal if the material is used in offshore applications, in particular in environments which comprise H2S. Normally it has been desired to avoid nitrogen in the steel material for offshore use e.g. as it is described in U.S. Pat. No. 6,408,891.